Viruses, particularly human retroviruses like the human immunodeficiency virus type 1 (HIV-1) or the human leukemia virus type I (HTLV-I), are the causative agents for very serious diseases. This is in the case of HIV-1 the Acquired Immune Deficiency Syndrome (AIDS) and in the case of HTLV-I Adult T-cell Leukemia (ATL) as well as noncancerous conditions known as Tropical Spastic Parapesis. HTLV-II is etiologically related to some cases of variant T-cell hairy cell leukemia. Both virus groups are dividing their replication cycle, similarly to the DNA viruses, in an "early" and a "late" stage of gene expression. The "early" phase of gene expression is characterized by the expression of the regulatory proteins, while in the "late" phase the structural proteins are synthesized.
The HTLV-I genome is coding for an activator of viral transcription termed Tax. The equivalent of Tax in HIV-1 is termed Tat. Tax and Tat appear to act primarily on the retroviral LTR (long terminal repeat) for viral gene expression. In addition, HTLV-I encodes an activator of viral structural gene expression termed Rex. A functional Rex protein is responsible for the increased transport of unspliced viral mRNA out of the nucleus into the cytoplasm of the infected cell. There these mRNA species are constituting the viral genome and encoding the structural proteins. Human Immunodeficiency Virus Type 1 (HIV-1) encodes a homologous protein termed Rev. The rev gene product is, as Rex in the HTLV-I system, absolutely required for the expression of the HIV-1 structural proteins.
The underlying reason for this is that the product of the rev gene (and its equivalents in other viral species) is having a dramatic effect on the selection of the splicing mode for the viral mRNA transcripts in infected cells. This effect is achieved in the case of Rev and Rex by posttranscriptional regulation, namely by enhancement of the transport into the cytoplasm of full-length mRNA transcripts, whereby expression of viral structural proteins such as Gag and Env for HIV-1 is initiated and expression of regulatory proteins is concomitantly suppressed (see e.g. for Rex M. Hidaka et al., EMBO J. 7 [1988] 519) or modulated (see e.g. for Rev M. H. Malim et al., Nature 335 [1988] 181). Thus Rev is not required for the expression of the fully spliced HIV-1 mRNAs encoding the viral regulatory proteins, including Tat and Rev.
In HIV-1 the selectivity of the induction noted above is due to an RNA target sequence required for Rev function termed Rev Response Element (RRE). RRE coincides with a large, 234 nucleotide RNA secondary structure present within the HIV-1 env gene. The equivalent structure in HTLV-I is termed Rex Response Element (RexRE or RRX). Rev appears to be the first protein which has been shown to regulate the nuclear export of RNA in a sequence specific manner.
Taking Rex as an illustration, the complete function of the Rex protein in regulating expression of the HTLV-I gag and env genes requires at least three functionally distinct component activities: nuclear and nucleolar localization, i.e. the capacity to be transported from the cytoplasmic site of synthesis of all proteins to the nucleus and there to be concentrated in the nucleolar region; specific recognition (directly or indirectly) of the RexRE (RRX) sequence in viral RNAs; and Rex effector activity, the presently still unknown activity of this regulatory protein which actually mediates export from the nucleus to the cytoplasm of partially spliced viral mRNA species that include the RexRE sequence.
Regarding the structural locations in the Rex protein where these component activities of the complete Rex function reside (i.e. the functional domains), all that was known prior to the present invention is that a positively charged peptide domain in the first twenty amino acids at the amino terminus of Rex is required for nucleolar localization (H. Siomi et al., Cell 55 [1988] 197-209).
As mentioned above both the rex gene product for HTLV-I and the rev gene product for HIV-1 are required for replication of the virus (see e.g. for HIV E. Terwilliger et al., J. Virol. 62, [1988] 655). The crucial importance of Rex and Rev is underscored by the fact that in spite of their different primary structures, they are related functionally, and HTLV-I Rex is able to exert its function in the other viral species, i.e. in HIV-1 (L. Rimsky et al., Nature 335 [1988] 738): thus even though
Rev and Rex do not share any significant homology on the nucleotide as well as on the amino acid level, PA0 the nucleotide sequences and stem and loop structures of the RRE differ from those of the RexRE (RRX) in HTLV-I, PA0 computer-generated prediction of secondary structures of the Rex and Rev proteins reveal no significant similarities and PA0 the Rex protein does not appear to bind to the same part of the RRE as the Rev protein does, PA0 A trans-dominant repressor of HIV-1 Rev function comprising a first and a second domain, the first domain having substantially the specific binding functions of wild-type HIV-1 Rev and the second.domain not having the activation functions of wild-type HIV-1 Rev, the second domain being modified from wild-type HIV-1 Rev by one or more mutations; preferably the first domain comprises from about amino acid position 10 to about amino acid position 68 of wild-type Rev and the modified second domain is derived from about amino acid position 68 to about amino acid position 90 of wild-type Rev; especially, the above one or more mutations are missense or deletion mutations which occur between about amino acid position 68 and about amino acid position 90, preferably from about 78 to about 86, especially from about 78 to about 83 or 84 of wild-type Rev, the specific binding functions of the first domain of wild-type HIV-1 Rev remaining substantially functionally intact; particularly the repressors pM10, pM21, pM22, pM27, pM28, pM29 and pM32 disclosed hereunder or a functional fragment or derivative thereof; PA0 a trans-dominant repressor of HIV-1 Rev function comprising a first domain having substantially the specific binding functions of wild-type HIV-1 Rev, this transdominant repressor not having the activation functions of wild-type HIV-1 Rev; preferably the first domain comprises from about amino acid position 10 to about amino acid position 68 of wild-type Rev and the transdominant repressor lacks from about amino acid position 68 to at least about amino acid position 90 of wild-type Rev; particularly the repressors p.DELTA.9/14 and p.DELTA.10/14 disclosed hereunder or a functional fragment or derivative thereof; PA0 a DNA segment that encodes a trans-dominant repressor of the function of the HTLV-I Rex protein, the repressor being modified from a wild-type form of the Rex protein by at least one trans-dominant negative mutation in the peptide domain of the wild-type Rex protein that exhibits the effector activity of the Rex protein, this repressor having substantially the nucleolar localization activity of the wild-type form of the Rex protein; preferably such a DNA segment in which the peptide domain of the wild-type Rex protein comprises from about amino acid position 59 to about amino acid position 121, especially in any one of the following amino acid positions: 59, 60, 64, 65, 119, 120 and 121; particularly a DNA segment comprising any of the following mutant rex genes: M6, M7, M13 and variants and derivatives thereof which exhibit trans-dominant repression of HTLV-I Rex protein function; PA0 a corresponding trans-dominant repressor of the function of the HTLV-I Rex protein so modified from a wild-type form, preferably having the ability to repress either the function of the HIV-1 Rev protein or the function of the HTLV-II Rex protein; PA0 a method for identifying a specific inhibitor of the gene activation function of the Rex protein comprising the steps of: PA0 an identification method as defined above wherein the decrease in the export of the mRNA that comprises the unused splice site is preferably detected by determining the level of production of a first protein, encoded by the mRNA that comprises the unused splice site, and the increase in export of the spliced form of the mRNA is preferably detected by determining the level of production of a second protein, encoded by the spliced form of the mRNA; PA0 an identification method as defined above wherein the mRNA comprising the regulatory response element and the splice site is encoded by a plasmid comprising the 3' end of an HTLV-I provirus including the coding regions for the Rex and Tax proteins, the complete env gene, the Rex response element and the entire 3' LTR; preferably by plasmid pgTAX-LTR disclosed hereunder; and the rex gene preferably is provided on plasmid pRex; PA0 plasmid pgTAX-LTR; PA0 a reagent kit for screening agents to identify a specific inhibitor of the gene activation function of the Rex protein according to the above identification method, comprising: PA0 a method of inhibiting replication of HIV-1, HTLV-I or HTLV-II comprising introducing a DNA segment as defined above into a cell having the ability to replicate one of these viruses and to express the DNA segment to produce a transdominant repressor of HTLV-I Rex function; and PA0 a method of inhibiting HIV-1, HIV-2 and SIV, especially HIV-1, replication comprising introducing into a cell infected with HIV-1 a trans-dominant repressor of HIV-1 Rev function.
it is nevertheless possible to substitute the Rev protein by the Rex protein in the HIV-1 system, and further, it has very recently been found that HTLV-I Rex and HIV-1 Rev can substitute for HIV-2 Rev (Rev2) and that HTLV-I Rex can also substitute for the analogous HTLV-II regulatory protein. This complementation is sufficient to rescue e.g. a rev-deficient HIV-1 provirus providing functional Rex protein in trans. On the other hand the reverse substitution to rescue a rex-deficient HTLV-I provirus by functional Rev protein does not seem to be feasible. Thus there is no complete symmetry in this respect. The basis for this lack of reciprocality is not yet understood, but it probably relates to differences in the functional aspects of these proteins that are required for target RNA sequence recognition.
Mutations in regulatory proteins may yield a gene product with a dominant negative phenotype over the wild-type function (I. Herskowitz, Nature 329 [1987] 317). Dominant negative mutant proteins, known as trans-dominant repressors, a small group of which have been discovered recently in several unrelated viruses, represent a novel class of anti-viral agents. In genetic analyses, negative mutations are those which cause a diminution or loss of a function of a gene. Dominant negative mutations are those that prevent other copies of the same gene, which have not been mutated (i.e. which have the wild-type sequence), from functioning properly. On the other hand recessive negative mutations do not so inhibit wild-type counterparts. Further, some dominant mutations inhibit wild-type genes only when the mutant and wild-type genes are located on the same chromosome (DNA or RNA molecule). In this case the inhibiting mutation is said to be "cis-acting". Alternatively, a dominant mutation may inhibit the corresponding wild-type gene even when located on a separate chromosome. This type is classified as a "trans-acting" dominant mutation or, more simply, as a transdominant mutation.
A few of these so-called transdominant genes have been described, concerning genes for eukaryotic or Herpes virus transcription factors (I. A. Hope and K. Struhl, Cell 46 [1986] 885; R. Gentz et al., Science 243 [1989] 1695; S. J. Triezenberg et al., Gen. & Devel. 2 [1988] 718; A. D. Friedman et al., Nature 335 [1988] 452). Thus, when overexpressed some deletion mutants of the Herpes simplex virus trans-activator VP16 inhibit VP16 function, thereby precluding replication of HSV-1 in normally permissive cells. As regards retroviruses, transdominant mutants have also been described, e.g. for the Tax protein of HTLV-II (W. Wachsman et al., Science 235 [1987] 674) and, after the priority date for the present invention, for the HIV-1 tat (M. Green et al., Cell 58 [1989] 215) and gag (D. Trono et al., Cell 59 [1989] 113) genes.
These differences in compositions and functions of these two regulatory proteins indicate that comparison of Rex structure with that of the Rev protein or its known mutants offers no guidance at all for selecting mutations that might produce trans-dominant repressors of the viral proteins.
A therapeutic application of the above concepts would involve the inhibition of production or overproduction of a deleterious gene product by manipulation of the gene to create dominant negative mutations whereby the resultant gene is encoding mutant regulatory proteins which when expressed disrupt the activity of the wild-type function (I. Herskowitz, Nature 329 [1987] 219). In the situation of viral, e.g. retroviral, infections it thus appears highly desirable to provide corresponding transdominant repressors of virus function by the construction of similar inhibitors of essential regulatory genes, e.g. inhibitors of the rev or rex gene. This approach would provide the requisite tools for "intracellular immunization", an approach to the treatment of viral infections first proposed in 1988 (D. Baltimore, Nature 335 [1988] 395).